Expansion of optical transport networks can be obtained by increasing the network capacity and/or reach. A higher network capacity is achieved by increasing the channel rate (time division multiplexing TDM) and increasing the density of channels in the respective transmission window, known as WDM (wavelength division multiplexing) and/or dense WDM (DWDM). The system reach, i.e. the distance between a transmitter and the next receiver, can be extended by optimizing the parameters of the transmission path.
However, optimizing the parameters of a transmission path is not a simple task. Optical signals suffer degradation along an optical path from such factors as loss, noise, inter-symbol interference, fiber dispersion, non-linearity of the elements and transmission medium, ambient temperature variations, etc.
A well-known solution to condition an optical signal is to use optical-electrical-optical (OEO) regenerators. Regeneration is the process of amplifying (correcting loss), reshaping (correcting noise and dispersion), retiming (synchronizing with the network clock), and retransmitting an optical signal. The regenerator design is channel-specific as the regeneration is performed in electrical format, so that the cost of the network increases dramatically as the channel density grows.
Emergence of optical amplifiers, which amplify all channels in the WDM signal by a certain gain in optical format (without OEO conversion), was essential in developing the D/WDM systems, as an optical amplifier may replace a number of regenerators, importantly reducing the cost of the network. Unfortunately, the optical amplifiers exhibit a wavelength-dependent gain profile, noise profile, and saturation characteristics. Hence, each optical channel experiences a different gain along a transmission link. The optical amplifiers also add noise to the signal, typically in the form of amplified spontaneous emission (ASE), so that the optical signal-to-noise ratio (OSNR) decreases at each amplifier site.
Furthermore, due to the intrinsic material characteristics of the transmission medium (the fiber), the channels of a WDM signal undergo different distortions and losses along the same link/path. To add to this, the individual performance of the channel transmitters and receivers is also different, so that each channel has different initial waveform distortions, and a different detection quality.
To summarize, the transfer function of optical amplifiers, transmission medium and other active and passive optical components in the optical link/path is a function of wavelength. This wavelength dependency on path configuration and optical device specifications results in a variable performance of the co-propagating channels at the receiving terminal for equal optical launched power levels. In other words, ‘not all wavelengths are created equal’. Experiments show that the ratio between the reach of the “best” and “worst” performing wavelengths can be more than 2:1.
Current transport networks are based on a WDM physical layer, using point-to-point (pt-pt) connectivity. As network flexibility is delivered electronically, termination of the photonic layer is necessary at each intermediate switching node along a path. In this type of network, channel allocation is fixed and link performance is optimized using span equalization.
There are numerous performance optimization methods applicable to traditional networks, all based on ‘equalizing’ a certain transmission parameter of the WDM signal, such as OSNR (optical signal-to-noise rate), BER (bit error rate), or Q-factor. It has been shown that the OSNR at the receiver can be equalized by adjusting the input optical power for all channels. For example, U.S. Pat. No. 5,225,922 (Chraplyvy et al.), issued on Jul. 6, 1993 to AT&T Bell Laboratories, provides for measuring the output SNRs and then iteratively adjusting the input powers to achieve equal SNRs. A telemetry path between the nodes provides the measurements obtained at one node to the other. Another example is U.S. Pat. No. 6,115,157 (Barnard et al.) issued to Nortel Networks Corporation on Sep. 5, 2000, which discloses a method of equalizing the channels of a WDM link based on an error threshold level for each channel in the WDM signal, the threshold being set in accordance with the channel rate. The transmitter power is adjusted taking into account the attenuations determined for all channels, which attenuations are calculated according to the measured BER.
Lately, ULR (ultra long reach) networks attempt to extend the distance traveled by the WDM channels in optical format for reducing the regeneration costs. To this end, ULR networks use a variety of techniques such as hybrid Raman-EDFA amplification, dispersion management, gain tilt control, etc. These techniques combined with other proprietary reach-increasing methods, have resulted in ULR networks where channels may travel over 3,000 km without regeneration.
However, current engineering methods based on span equalization present numerous inherent disadvantages.
1. Electrical switch based flexibility limits the reach of an optical channel to the distance between two consecutive switching nodes. Thus, channels that can travel farther are truncated by the geography of the network. Since the distance between most nodes is in practice under 1,000 km, only links that are over 1,000 km (i.e. approximately less than 20-30% of all links) can benefit of any reach optimization technique. On the other hand, the majority of connections (end-to-end signal paths) exceed the nodal spacing of 1,000 km. Truncation of 70% of the connections at the switching nodes significantly reduces the benefits of the ULR techniques.
2. Traditional point-to-point WDM networks perform span and link engineering based on the “worst case” rules. Thus, in addition to the reach truncation described at 1), performance of channels that can travel farther is further lowered to the performance of the weakest channel/s. This is clearly not the most advantageous way of using the network resources.
3. Point-to-point networks are deployed based on engineering estimates and component specified limits, rather than on measured data, which in many occasions significantly under/over approximate the real span/link performance capabilities. When over-estimated, a span/link must be re-engineered after an unsuccessful set-up attempt. When under-estimated, the equipment is used inefficiently.
4. To make-up for an eventual over-estimation, current estimates are complemented with a number of engineering margins that further limit the reach.
5. As a typical optical network is characterized by a different loss in each span depending upon the fiber type, fiber length, cabling and slicing losses, span equalization becomes a span-specific, complex operation. In addition, different network operators have distinctive losses and loss distribution requirements in their networks. All this results in a plurality of hardware variants for each section of a network, with the ensuing complexity in inventory management and additional costs.
There is a need to provide a WDM network with a method for reach-capacity optimization, which uses the network resources efficiently.